Highway 9 was first assumed into the provincial highway system on February 26, 1920 as the Arthur–Kincardine Road. It was extended to Cookstown in the early 1930s via Orangeville and Shelburne, creating a short lived concurrency with Highway 10. In 1937, the road between Orangeville and Schomberg was designated part of Highway 9. The concurrency was discontinued, and the remainder became Highway 89. In 1965, Highway 9 was extended to Newmarket along Davis Drive.

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Highway 9 begins at the edge of Kincardine near the eastern shoreline of Lake Huron. The roadway continues west past Highway 21 to the lake as Broadway Street. To the east of Highway 21, the highway travels along a concession road for 40 km (25 mi), through the Saugeen Conservation Lands to the town of Walkerton. Within Walkerton, Highway 9 turns south at a junction with Bruce County Road 4. It travels south to the village of Mildmay, where it curves to the south-east. The highway continues in this direction through the villages of Clifford and Harriston, forming the centre of a thin band of farmland oriented at a 45-degree angle to the surrounding land. In the centre of Harriston, Highway 9 ends at a four way junction. Highway 89 travels north-east from this location, while Highway 23 travels south-west. Highway 9 formerly continued south-west, but is now known as Wellington County Road 109 past this junction.

At Highway 10 in Orangeville, Highway 9 resumes and travels east to Highway 400, crossing the Niagara Escarpment along the way. The highway once continued east to Yonge Street in Newmarket, but this section is known as York Regional Road 31. Highway 9 ends at an interchange with Highway 400. A commuter parking lot is provided for carpooling.

The portion of Highway 9 between Kincardine and the junction with Yonge Street in Walkerton was built originally as the Durham Settlement Road or Durham Road for short. The Durham Road was surveyed and constructed between 1849 and 1851. It extended from the border between present-day Grey Highlands, Grey County and Clearview, Simcoe County, south of Singhampton, through Flesherton (on the then Toronto–Sydenham Settlement Road, today Highway 10), Durham (on the Garafraxa Settlement Road, today Highway 6, and one reason the road took its name), Hanover and Walkerton to Kincardine.[3][4]
The western section from Kincardine to Walkerton is today Highway 9, and the other sections to the east were at one point part of Highway 4, and today Bruce County Road 4 and Grey County Road 4.[5]

On February 26, 1920, the Arthur–Kincardine Road was designated as a provincial highway. It connected what would become Highway 6 with Kincardine, on the shores of Lake Huron. In August 1925, the road was numbered as Highway 9, alongside the other existing provincial highways. The route was extended to Cookstown in the early 1930s. The road between Arthur and Orangeville was assumed as part of Highway 9 on March 12, 1930; the road between Shelburne and Cookstown was assumed on May 27, 1931.[6]
The two roads were connected by creating a concurrency along Highway 10. On February 10, 1937, the road between Orangeville and Schomberg was designated part of Highway 9.[7]
To alleviate the forked path of the highway, the concurrency with Highway 10 was discontinued and the road between Shelburne and Cookstown was renumbered as Highway 89. By October 1963, Davis Drive was built west of Newmarket, across the Holland Marsh to Schomberg. On July 23, 1965, Highway 9 was extended to Newmarket along Davis Drive, bringing its total length to 191.7 km (119.1 mi).[8]

A long-standing issue through most of the history of Highway 9 is the Orangeville Bypass. In the 1960s, the Highway 10 bypass was constructed. At the same time, Highway 9 was rerouted from its straight route to meet the new bypass, creating Buena Vista Drive as a result. Starting in 1978, numerous plans were formulated for a southern bypass of Broadway, none of which came to fruition.[9][10]
Orangeville eventually resorted to constructing the road themselves, completing several kilometres before local Member of Provincial Parliament and premier Ernie Eves contributed C$7 million of provincial funding to the project.[11]
The 6.8 km (4.2 mi) bypass was finally opened to traffic on August 3, 2005.[12]

On January 1, 1998, the province transferred sections of Highway 9 between Harriston and Orangeville to Dufferin County and Wellington County, creating a 67.7 km (42.1 mi) gap between sections of the highway. This transfer has been widely contested since it took place, often used as an example for the hastily executed highway transfers in Ontario.[13][14]
On September 1, 1999, the Regional Municipality of York assumed responsibility for the section of Highway 9 between Highway 400 and Yonge Street.[15]

^For a 19th century historic map showing the route through Kincardine Township (the present day municipality of Kincardine), see "Township of Kincardine". Canadian County Atlas Project. McGill University. 2001. Retrieved September 8, 2011. Other historic township maps showing the route are available at the same site.